The invention relates to the coating of polymer parts and, in particular, to enhancing the adhesion of coatings such as a metallic coating or paint to a polymer substrate or a polymer composite substrate by anodically assisted etching of the substrate prior to application of a coating.
Due to their low cost and ease of processing/shaping by various means, polymeric materials, which are optionally filled with or reinforced with electrically conductive materials selected from the group of metals, metal alloys, and carbon based materials selected from the group consisting of graphite, graphite fibers, carbon and carbon nanotubes, are widely used.
Metal coating of polymer parts is of considerable commercial importance because of the desirable properties obtained by combining polymers and metallic materials. In many applications filled and reinforced polymers require a metallic coating applied by electroless deposition techniques and/or electrodeposition. The metallic deposits must adhere well to an underlying polymer substrate even in corrosive environments and when subjected to thermal cycling, such as are encountered in outdoor service.
The prior art describes numerous processes for metalizing polymers to render them suitable for electroplating by conditioning a polymer substrate's surface to ensure that metallic deposits adequately bond to the surface resulting in durable and adherent metallic coatings. The most popular substrate conditioning/activation process is chemical etching. The following patent is representative of the prior art.
Stevenson in U.S. Pat. No. 4,552,626 (1985) describes a process for metallic plating filled thermoplastic resins such as filled Nylon-6®. The filled resin surface to be plated is cleaned and rendered hydrophilic and preferably deglazed by a suitable solvent or acid. At least a portion of the filler at a surface is removed, preferably by a suitable acid. Thereafter electroless plating is applied to the surface to provide an electrically conductive metallic deposit followed by applying at least one metallic layer by electroplating to provide a desired wear resistant and/or decorative metallic surface.
Permanganate etch solutions, e.g. alkaline permanganate solutions, e.g. potassium permanganate solutions, are commonly used for etching laminated circuit boards prior to copper plating in the electronics industry. Typically a 3-stage process is followed which includes: a) solvent conditioning (swelling), b) oxidative etch, and c) neutralization. In order to successfully etch the resin in the laminate board, an effective solvent must be used in the conditioning step to penetrate and swell the resin surface. This is believed to weaken the cross-link bonds in the resin and result in an increased resin removal rate in the oxidative (permanganate) etch step. The toxicity of the chromium compounds and their potential hazards as water pollutants and the safety precautions needed with sulfuric acid have increased the commercial use of permanganate solutions, particularly alkaline permanganate solutions. The following patent illustrates such use.
Leech in U.S. Pat. No. 4,054,693 (1977) discloses processes for the activation of resinous materials with a composition comprising water, permanganate ion and manganate ion at a pH in the range of 11 to 13 to provide superior peel strength following electroless metal deposition.
While acidic and alkaline permanganate solutions are very active they are, however, not very stable and rapidly decompose. The operating life of a permanganate etchant bath can be relatively limited as permanganate ions are reduced during the etching process to manganese species of lower oxidation states, such as manganate and manganese dioxide. This reduction results directly from the etching process as well as from the etchant bath conditions; for instance, an alkaline bath promotes permanganate disproportionation to yield manganate. As it is permanganate rather than manganese species of lower oxidation state which exhibit powerful polymer etching properties, the bath must either be regularly replaced with fresh permanganate solution or supplemented with additional permanganate ions to maintain etchant activity. Preferably, the permanganate concentration is maintained by oxidation of reduced manganese species present in the bath as addition of new permanganate to an existing bath or bath replacement are both expensive and burdensome. The following patent illustrates such oxidation.
Courduvelis in U.S. Pat. No. 4,592,852 (1986) describes an improved alkaline permanganate composition for etching printed circuit boards by incorporating a secondary oxidant in the solution, capable of oxidizing manganate ion to permanganate ion, such as sodium hypochlorite, chlorine, bromine, ozone, hypochlorite salts, metaperiodate salts, trichloro-s-triazinetrione and its salts, and the like.
A convenient method of permanganate regeneration is oxidative electrolysis, as is generally described by Sullivan in U.S. Pat. No. 4,859,300 (1989). Sullivan describes a permanganate process for treating plastics, e.g., printed circuit boards, to enhance the adhesion of metal plating to polymers. Electrolysis is employed to either maintain a certain permanganate level in the operating bath or to regenerate a spent or used bath. The efficiency of such electrolysis is limited by reduction reactions occurring at the cathode, specifically the reduction of permanganate and lower oxidative state manganese compounds. Reduction yielding manganese dioxide particularly limits cell efficiency. Manganese dioxide is extremely insoluble in typical etching solutions and thus, once formed, cannot be oxidized to permanganate at the anode.
Carbon-fibers and/or graphite-fibers are a popular choice for reinforcing polymers and to increase their electrical conductivity. A number of electrochemical processes are known to roughen the carbon/graphite-fiber surface to improve the adhesion between the fiber and the polymer it reinforces. Patents illustrating this is described below.
Ray in U.S. Pat. No. 3,671,411 (1972) describes the activation of carbon fibers or yarn as used in fiber reinforced composites by electrolytically treating them to improve the surface characteristics and thereby to improve fiber or yarn bonding or adhesion to a matrix material. The fiber or yarn forms the anode and a suitable electrolyte is an aqueous caustic solution.
Reith in U.S. Pat. No. 5,203,973 (1973) discloses a method to roughen the surface of carbon to improve adhesion between the metallic coating and a carbon part. Carbon parts are submersed in an aqueous solution of sodium hydroxide or potassium hydroxide and an anodic current is applied for two or three minutes.
The following patents illustrate the provision of a fine-grained metallic coating on a polymeric substrate.
Palumbo in U.S. Pat. No. 7,320,832 (2008) discloses means for matching the coefficient of thermal expansion (CTE) of a fine-grained metallic coating to the CTE of the substrate by adjusting the composition of the alloy and/or by varying the chemistry and volume fraction of particulates embedded in the coating. The fine-grained metallic coatings are particularly suited for strong and lightweight articles, precision molds, sporting goods, automotive parts and components exposed to thermal cycling and include polymeric substrates optionally reinforced with conductive fibers. The low CTEs and the ability to match the CTEs of the fine-grained metallic coatings with the CTEs of the substrate minimize dimensional changes during thermal cycling and prevent delamination.
Palumbo in U.S. Pat. No. 7,354,354 (2008) discloses lightweight articles comprising a polymeric material at least partially coated with a fine-grained metallic material. The fine-grained metallic material has an average grain size of 2 nm to 5,000 nm, a thickness between 25 micron and 5 cm, and a hardness between 200 VHN and 3,000 VHN. The lightweight articles are strong and ductile and exhibit high coefficients of restitution and a high stiffness and are particularly suitable for a variety of applications including aerospace and automotive parts, sporting goods, and the like.